DKFZ. Germany: In the bone marrow, blood stem cells give rise to a large variety of mature blood cells via progenitor cells at various stages of maturation. Scientists from the German Cancer Research Center (DKFZ) have developed a way to equip mouse blood stem cells with a fluorescent marker that can be switched on from the outside. Using this tool, they were able to observe, for the first time, how stem cells mature into blood cells under normal conditions in a living organism. With these data, they developed a mathematical model of the dynamics of hematopoiesis. The researchers have now reported in the journal Nature that the normal process of blood formation differs from what scientists had previously assumed when using data from stem cell transplantations.
Since ancient times, humankind has been
aware of how important blood is to life. Naturalists speculated for
thousands of years on the source of the body’s blood supply. For several
centuries, the liver was believed to be the site where blood forms. In
1868, however, the German pathologist Ernst Neumann discovered immature
precursor cells in bone marrow, which turned out to be the actual site
of blood cell formation, also known as hematopoiesis. Blood formation
was the first process for which scientists formulated and proved the
theory that stem cells are the common origin that gives rise to various
types of mature cells.
“However, a problem with almost all research
on hematopoiesis in past decades is that it has been restricted to
experiments in culture or using transplantation into mice,” says
Professor Hans-Reimer Rodewald from the German Cancer Research Center
(Deutsches Krebsforschungszentrum, DKFZ). “We have now developed the
first model where we can observe the development of a stem cell into a
mature blood cell in a living organism.”
Dr. Katrin Busch from Rodewald’s team developed
genetically modified mice by introducing a protein into their blood stem
cells that sends out a yellow fluorescent signal. This fluorescent
marker can be turned on at any time by administering a specific reagent
to the animal. Correspondingly, all daughter cells that arise from a
cell containing the marker also send out a light signal.
When Busch turned on the marker in adult animals,
it became visible that at least one third (approximately 5000 cells) of a
mouse’s hematopoietic stem cells produce differentiated progenitor
cells. “This was the first surprise,” says Busch. “Until now, scientists
had believed that in the normal state, very few stem cells – only about
ten – are actively involved in blood formation.”
However, it takes a very long time for the
fluorescent marker to spread evenly into peripheral blood cells, an
amount of time that even exceeds the lifespan of a mouse. Systems
biologist Prof. Thomas Höfer and colleagues (also of the DKFZ) performed
mathematical analysis of these experimental data to provide additional
insight into blood stem cell dynamics. Their analysis showed that,
surprisingly, under normal conditions, the replenishment of blood cells
is not accomplished by the stem cells themselves. Instead, they are
actually supplied by first progenitor cells that develop during the
following differentiation step. These cells are able to regenerate
themselves for a long time – though not quite as long as stem cells do.
To make sure that the population of this cell type never runs out, blood
stem cells must occasionally produce a couple of new first progenitors.
During embryonic development of mice, however, the
situation is different: To build up the system, all mature blood and
immune cells develop much more rapidly and almost completely from stem
cells.
The investigators were also able to accelerate
this process in adult animals by artificially depleting their white
blood cells. Under these conditions, blood stem cells increase the
formation of first progenitor cells, which then immediately start
supplying new, mature blood cells. In this process, several hundred
times more cells of the so-called myeloid lineage (thrombocytes,
erythrocytes, granulocytes, monocytes) form than long-lived lymphocytes
(T cells, B cells, natural killer cells) do.
“When we transplanted our labeled blood stem cells
from the bone marrow into other mice, only a few stem cells were active
in the recipients, and many stem cells were lost,” Rodewald explains.
“Our new data therefore show that the findings obtained up until now
using transplanted stem cells can surely not be reflective of normal
hematopoiesis. On the contrary, transplantation is an exception [to the
rule]. This shows how important it is that we actually follow
hematopoiesis under normal conditions in a living organism.”
The scientists in Rodewald’s department, working
together with Thomas Höfer, now also plan to use the new model to
investigate the impact of pathogenic challenges to blood formation: for
example, in cancer, cachexia or infection. This method would also enable
them to follow potential aging processes that occur in blood stem cells
in detail as they occur naturally in a living organism.
Katrin Busch, Kay Klapproth, Melania Barile,
Michael Flossdorf, Tim Holland-Letz, Susan M. Schlenner, Michael Reth,
Thomas Höfer and Hans-Reimer Rodewald. Fundamental properties of
unperturbed haematopoiesis from stem cells in vivo. Nature 2015,
DOI:10.1038/nature14242